Agriculture Reference
In-Depth Information
of multiple transgenes in a single plant genomic
location (Ow 2005).
Over the last decade, efforts have been made
to develop methods for the direct transformation
of plant germline cells, such as microspores,
ovules, or meristem cells, which develop directly
into differentiated plant tissues and ultimately
seeds. Limited success was achieved until it was
demonstrated in Arabidopsis that transformants
could be generated by germinating seeds in the
presence of
Agrobacterium
(Feldmann and Marks
1987), and later that infi ltration of fl oral tissues
with
Agrobacterium
could also produce transfor-
mants (Bechtold et al., 1993). These are im-
portant developments because transformation
methods via germ-line tissues are potentially
more effi cient, less labor-intensive, less depen-
dent on tissue genotype for regeneration cap-
ability, and not prone to the somaclonal variation
that can affect culture-based transgenics.
Recently, successful applications of germ-line
transformation methods were reported in wheat
(Supartana et al., 2006; Zale and Steber 2006).
Despite the advantages of
in planta
transforma-
tion methods, it is too early to say what effi ciency
levels will be routinely achieved by using them
in wheat.
The new standard for transgenic crops is likely
to include the absence of antibiotic- and herbi-
cide-resistant selection genes that are remnants
of the transformation process. Particularly for
applications destined for release into the
commercial marketplace (other than herbicide
resistance), the preference clearly exists among
consumers, food manufacturers, and environ-
mentalists for the absence of selection genes and
their products in the food supply (Natarajan and
Turna 2007).
When transgenic wheat plants will fi nd a way
into US commercial production is diffi cult to
predict. Although transgenic maize, soybean,
and cotton (
Gossypium hirsutum
L.) have been
planted and harvested for at least a decade without
adverse effects on either the food supply or the
environment, opposition to transgenic wheat is
high, particularly in Japan and Europe. For many
persons, “genetically modifi ed” wheat and rice
are especially problematic because these grains
comprise a signifi cant percentage of the human
diet.
Assuming barriers to commercial production
of transgenic wheat plants are eventually over-
come, what traits are the most likely targets for
improvement? For producers, resistance to
Fusarium head blight, especially to initial infec-
tion (Type I resistance), would be very useful to
combine with existing traditional sources of Type
II resistance. Broad-spectrum resistance to wheat
rusts would also be valuable. Drought and/or salt
tolerance could expand the growing areas of wheat
to more marginal land. Improving the suitability
of wheat straw for biofuel production could
expand marketplace demand for wheat. Improve-
ments in grain nutritional composition—such as
increases in the quantity and quality of protein
and increased levels or bioavailability of iron—
would be of direct benefi t to consumers. Some
have proposed using biotechnology to reduce the
allergenicity of wheat for persons with celiac
disease, but we consider this application unlikely
to be successful, because many different classes of
wheat seed proteins, including several that are
important for wheat breadmaking quality, contain
peptides that are toxic to celiac patients (Hamer
2005; Howdle 2006).
Bringing a genetically engineered crop plant
into the marketplace requires considerable
resource investment in terms of generating large
numbers of plants, characterizing them exten-
sively over several generations, securing intel-
lectual property, and safety testing each transgenic
line before release. The fi rst two processes are
similar to their counterparts in traditional breed-
ing, but the latter two are much more costly for
transgenic than for traditionally bred cultivars.
At this time, relatively few traits are considered
to have suffi cient value to undertake commercial
development of transgenic wheat, especially
when the additional costs of seed segregation
and identity preservation in the marketplace
are considered (Wilson et al., 2003; Johnson
et al., 2005). Nevertheless, the rapid increase in
worldwide areas planted with genetically engi-
neered maize, cotton, and soybean cultivars
during the fi rst decade of their availability is
proof that such traits are of value to producers
